Yes, it is a valid primary diagnosis, but I would question whether or not it is the correct code. A screening, in ICD-10 terms at least, means that the patient is having the test solely as a preventive measure - in other words, they do not have any symptoms of the disease, they have not had contact with any sick persons or persons suspected of having the disease, no exposure or other reason to need the test. We are not really at the phase of this disease yet where we are beginning to conduct screenings of the healthy population.
But if in fact the code is correct, then the denial is probably correct also. All insurance plans have very specific and limited coverage for preventive services listed in the patients' plans, so I would not be surprised at this early stage in the disease, if the test would only be covered for patients who are symptomatic or who, due to exposure, have a demonstrated medical need for the test. If a screening is not a covered benefit for the patient, then it's the patient's responsibility - it's not due to a problem with the coding.